The Accuracy of a Computer

Craig Spears

November 9, 2006

With so many numbers that computer simulation programs can throw at you how do you know what numbers actually mean anything? One thing you have that a computer doesn’t is common sense. A computer knows only what it’s told, and for many programs it’s told to spew forth the total number of numbers that it can.

Here’s an example: 777.0270823938 sgpm. Now, my challenge for you is to go out to your plant and get a flow rate of exactly 777.0270823938 sgpm. For the computer it’s easy to give an output with 12 decimal places – all it is doing is simple addition (very quickly). However, most of those decimal places are meaningless to you in the field.

Now, the numbers are not completely meaningless. The computer uses these to verify that it has all of the mass or heat balances in check, and it gives very good balances because of all of these extra numbers. In the case of ProMax, it shows what type of error is remaining on each of the steps it is computing; normally it drives this to approximately an error of 1x10-6 (0.000001 or 0.0001%). This type of accuracy in the balances reassures me in knowing that the numbers are good to use, but all of these numbers have their place. I need to know that expecting an operator to have the plant running at all of those 12 decimal places is unrealistic at best.